how do I replenish troops?

[Charles.]

Ive only ever played empire and napoleon so now I have no idea how I replenish my troops I know you have to retrain them at settlements but I took jeruselum and I can only retrain certain units within this settlement???

[John Doe]

You can only retrain unit that you can get from that settlements. For example, you don't get heavy cavalry from city, so you must be in a castle to be able to retrain it. And you also need to have the right building for it: DFK are only available from a fortress, so a castle won't be allow to replenish the unit, and sergent spearmen you need to have the 2400 gold barrack built.

Mercenary are different, you can retrain them as long as they are recruitable in the region, so if you got to Jerusalem with mercenary crossbow, you will not be able to retrain them. Units only available to a general on a crusade are not retrainable (fanatics, crusader knights...)

[rubenandthejets]

That's right-you can only retrain troops using the same kind of building you used to recruit them initially. Mailed knights can only be retrained in a castle for example.

You should be able to retrain some of the millitia style units you initially recruited in a city soon. Just after you take a town or castle it takes a couple of turns before units become available.

Mercinaries can be retrained in the same kind of regions you initially recruited them, eg Slav archers in the Balkans region, Cuman horse archers in Russia, Turkomans in Asia Minor and Welsh longbows and spears in Wales and England. The crossbowmen and spears have the biggest recruitment / replenishment areas as they are can be found in most of western Europe. I've never been able to retrain the units available to Crusader armies except by merging them together.

Lastly, some merc. units go out of fashion, so in mid game there aren't any more crossbowmen to replenish my old units as the mercs start using pavises.

To solve your immediate problem, pop your general outside the city and recruit some local mercs. If there are none available in Jerusalem, try a neighbouring province. You should be able to retrain them in the city if they get banged up.